Influenza virus causes worldwide seasonal infections and occasional pandemics with high mortality rate. Human viruses are known to have a preference for ?2-6 linked sialic acids (NeuAc?2-6Gal), while avian viruses exhibit a preference for ?2-3 linked sialic acids (NeuAc?2-3Gal). This difference in receptor specificity is widely considered a major species barrier for transmission of avian viruses in the human population. Although this binary model of receptor specificity has been useful, it belies the true complexity of sialic acid containing glycans on host cells and has becoming increasingly limiting since the receptor binding properties of influenza viruses continue to evolve. The use of glycan microarrays with dozens of ?2-3 and ?2-6 linked glycans has shown that influenza viruses evolve by restricting their specificity to specific glycans within those broad groups. However, interpretation of these findings for their relevance to influenza biology is uncertain since there is little information about the types of glycans that are actually present on human airway epithelium, and whether relevant airway epithelium glycans are represented on glycan microarrays. This project aims to identify the glycan structures on human airway epithelial cells that bind human influenza virus and expand glycan array libraries to include them, 2) to determine how the specificity and activity of human influenza virus hemagglutinins (HAs) and neuraminidases (NAs) evolve under immune selective pressure to retain their ability to interact with human airway receptors and 3) to use the information gleaned about receptor specificity to develop reliable methods for analysis and propagation of influenza in the laboratory. This information will identify receptor determinants on the human airway that are shared by human influenza viruses, and shed light on properties of the HA and NA that contribute to pandemic risk of influenza viruses from avian viruses that occasionally infect humans.